The Go Programming Language Specification

Version of May 9, 2018

Introduction

This is a reference manual for the Go programming language. For
more information and other documents, see golang.org.

Go is a general-purpose language designed with systems programming
in mind. It is strongly typed and garbage-collected and has explicit
support for concurrent programming. Programs are constructed from
packages, whose properties allow efficient management of
dependencies.

The grammar is compact and regular, allowing for easy analysis by
automatic tools such as integrated development environments.

Lower-case production names are used to identify lexical tokens.
Non-terminals are in CamelCase. Lexical tokens are enclosed in
double quotes "" or back quotes ``.

The form a … b represents the set of characters from
a through b as alternatives. The horizontal
ellipsis … is also used elsewhere in the spec to informally denote various
enumerations or code snippets that are not further specified. The character …
(as opposed to the three characters ...) is not a token of the Go
language.

Source code representation

Source code is Unicode text encoded in
UTF-8. The text is not
canonicalized, so a single accented code point is distinct from the
same character constructed from combining an accent and a letter;
those are treated as two code points. For simplicity, this document
will use the unqualified term character to refer to a Unicode code point
in the source text.

Each code point is distinct; for instance, upper and lower case letters
are different characters.

Implementation restriction: For compatibility with other tools, a
compiler may disallow the NUL character (U+0000) in the source text.

Implementation restriction: For compatibility with other tools, a
compiler may ignore a UTF-8-encoded byte order mark
(U+FEFF) if it is the first Unicode code point in the source text.
A byte order mark may be disallowed anywhere else in the source.

Characters

The following terms are used to denote specific Unicode character classes:

In The Unicode Standard 8.0,
Section 4.5 "General Category" defines a set of character categories.
Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo
as Unicode letters, and those in the Number category Nd as Unicode digits.

Lexical elements

Comments

Line comments start with the character sequence //
and stop at the end of the line.

General comments start with the character sequence /*
and stop with the first subsequent character sequence */.

A comment cannot start inside a rune or
string literal, or inside a comment.
A general comment containing no newlines acts like a space.
Any other comment acts like a newline.

Tokens

Tokens form the vocabulary of the Go language.
There are four classes: identifiers, keywords, operators
and punctuation, and literals. White space, formed from
spaces (U+0020), horizontal tabs (U+0009),
carriage returns (U+000D), and newlines (U+000A),
is ignored except as it separates tokens
that would otherwise combine into a single token. Also, a newline or end of file
may trigger the insertion of a semicolon.
While breaking the input into tokens,
the next token is the longest sequence of characters that form a
valid token.

Semicolons

The formal grammar uses semicolons ";" as terminators in
a number of productions. Go programs may omit most of these semicolons
using the following two rules:

When the input is broken into tokens, a semicolon is automatically inserted
into the token stream immediately after a line's final token if that token is

Integer literals

An integer literal is a sequence of digits representing an
integer constant.
An optional prefix sets a non-decimal base: 0 for octal, 0x or
0X for hexadecimal. In hexadecimal literals, letters
a-f and A-F represent values 10 through 15.

Floating-point literals

A floating-point literal is a decimal representation of a
floating-point constant.
It has an integer part, a decimal point, a fractional part,
and an exponent part. The integer and fractional part comprise
decimal digits; the exponent part is an e or E
followed by an optionally signed decimal exponent. One of the
integer part or the fractional part may be elided; one of the decimal
point or the exponent may be elided.

Rune literals

A rune literal represents a rune constant,
an integer value identifying a Unicode code point.
A rune literal is expressed as one or more characters enclosed in single quotes,
as in 'x' or '\n'.
Within the quotes, any character may appear except newline and unescaped single
quote. A single quoted character represents the Unicode value
of the character itself,
while multi-character sequences beginning with a backslash encode
values in various formats.

The simplest form represents the single character within the quotes;
since Go source text is Unicode characters encoded in UTF-8, multiple
UTF-8-encoded bytes may represent a single integer value. For
instance, the literal 'a' holds a single byte representing
a literal a, Unicode U+0061, value 0x61, while
'ä' holds two bytes (0xc30xa4) representing
a literal a-dieresis, U+00E4, value 0xe4.

Several backslash escapes allow arbitrary values to be encoded as
ASCII text. There are four ways to represent the integer value
as a numeric constant: \x followed by exactly two hexadecimal
digits; \u followed by exactly four hexadecimal digits;
\U followed by exactly eight hexadecimal digits, and a
plain backslash \ followed by exactly three octal digits.
In each case the value of the literal is the value represented by
the digits in the corresponding base.

Although these representations all result in an integer, they have
different valid ranges. Octal escapes must represent a value between
0 and 255 inclusive. Hexadecimal escapes satisfy this condition
by construction. The escapes \u and \U
represent Unicode code points so within them some values are illegal,
in particular those above 0x10FFFF and surrogate halves.

After a backslash, certain single-character escapes represent special values:

String literals

A string literal represents a string constant
obtained from concatenating a sequence of characters. There are two forms:
raw string literals and interpreted string literals.

Raw string literals are character sequences between back quotes, as in
`foo`. Within the quotes, any character may appear except
back quote. The value of a raw string literal is the
string composed of the uninterpreted (implicitly UTF-8-encoded) characters
between the quotes;
in particular, backslashes have no special meaning and the string may
contain newlines.
Carriage return characters ('\r') inside raw string literals
are discarded from the raw string value.

Interpreted string literals are character sequences between double
quotes, as in "bar".
Within the quotes, any character may appear except newline and unescaped double quote.
The text between the quotes forms the
value of the literal, with backslash escapes interpreted as they
are in rune literals (except that \' is illegal and
\" is legal), with the same restrictions.
The three-digit octal (\nnn)
and two-digit hexadecimal (\xnn) escapes represent individual
bytes of the resulting string; all other escapes represent
the (possibly multi-byte) UTF-8 encoding of individual characters.
Thus inside a string literal \377 and \xFF represent
a single byte of value 0xFF=255, while ÿ,
\u00FF, \U000000FF and \xc3\xbf represent
the two bytes 0xc30xbf of the UTF-8 encoding of character
U+00FF.

If the source code represents a character as two code points, such as
a combining form involving an accent and a letter, the result will be
an error if placed in a rune literal (it is not a single code
point), and will appear as two code points if placed in a string
literal.

A constant value is represented by a
rune,
integer,
floating-point,
imaginary,
or
string literal,
an identifier denoting a constant,
a constant expression,
a conversion with a result that is a constant, or
the result value of some built-in functions such as
unsafe.Sizeof applied to any value,
cap or len applied to
some expressions,
real and imag applied to a complex constant
and complex applied to numeric constants.
The boolean truth values are represented by the predeclared constants
true and false. The predeclared identifier
iota denotes an integer constant.

In general, complex constants are a form of
constant expression
and are discussed in that section.

Numeric constants represent exact values of arbitrary precision and do not overflow.
Consequently, there are no constants denoting the IEEE-754 negative zero, infinity,
and not-a-number values.

An untyped constant has a default type which is the type to which the
constant is implicitly converted in contexts where a typed value is required,
for instance, in a short variable declaration
such as i := 0 where there is no explicit type.
The default type of an untyped constant is bool, rune,
int, float64, complex128 or string
respectively, depending on whether it is a boolean, rune, integer, floating-point,
complex, or string constant.

Implementation restriction: Although numeric constants have arbitrary
precision in the language, a compiler may implement them using an
internal representation with limited precision. That said, every
implementation must:

Represent integer constants with at least 256 bits.

Represent floating-point constants, including the parts of
a complex constant, with a mantissa of at least 256 bits
and a signed binary exponent of at least 16 bits.

Give an error if unable to represent an integer constant
precisely.

Give an error if unable to represent a floating-point or
complex constant due to overflow.

Round to the nearest representable constant if unable to
represent a floating-point or complex constant due to limits
on precision.

These requirements apply both to literal constants and to the result
of evaluating constant
expressions.

Variables

A variable is a storage location for holding a value.
The set of permissible values is determined by the
variable's type.

Structured variables of array, slice,
and struct types have elements and fields that may
be addressed individually. Each such element
acts like a variable.

The static type (or just type) of a variable is the
type given in its declaration, the type provided in the
new call or composite literal, or the type of
an element of a structured variable.
Variables of interface type also have a distinct dynamic type,
which is the concrete type of the value assigned to the variable at run time
(unless the value is the predeclared identifier nil,
which has no type).
The dynamic type may vary during execution but values stored in interface
variables are always assignable
to the static type of the variable.

A variable's value is retrieved by referring to the variable in an
expression; it is the most recent value
assigned to the variable.
If a variable has not yet been assigned a value, its value is the
zero value for its type.

Types

A type determines a set of values together with operations and methods specific
to those values. A type may be denoted by a type name, if it has one,
or specified using a type literal, which composes a type from existing types.

Each type T has an underlying type: If T
is one of the predeclared boolean, numeric, or string types, or a type literal,
the corresponding underlying
type is T itself. Otherwise, T's underlying type
is the underlying type of the type to which T refers in its
type declaration.

type (
A1 = string
A2 = A1
)
type (
B1 string
B2 B1
B3 []B1
B4 B3
)

The underlying type of string, A1, A2, B1,
and B2 is string.
The underlying type of []B1, B3, and B4 is []B1.

Method sets

A type may have a method set associated with it.
The method set of an interface type is its interface.
The method set of any other type T consists of all
methods declared with receiver type T.
The method set of the corresponding pointer type*T
is the set of all methods declared with receiver *T or T
(that is, it also contains the method set of T).
Further rules apply to structs containing embedded fields, as described
in the section on struct types.
Any other type has an empty method set.
In a method set, each method must have a
unique
non-blankmethod name.

The method set of a type determines the interfaces that the
type implements
and the methods that can be called
using a receiver of that type.

Boolean types

A boolean type represents the set of Boolean truth values
denoted by the predeclared constants true
and false. The predeclared boolean type is bool;
it is a defined type.

Numeric types

uint8 the set of all unsigned 8-bit integers (0 to 255)
uint16 the set of all unsigned 16-bit integers (0 to 65535)
uint32 the set of all unsigned 32-bit integers (0 to 4294967295)
uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615)
int8 the set of all signed 8-bit integers (-128 to 127)
int16 the set of all signed 16-bit integers (-32768 to 32767)
int32 the set of all signed 32-bit integers (-2147483648 to 2147483647)
int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807)
float32 the set of all IEEE-754 32-bit floating-point numbers
float64 the set of all IEEE-754 64-bit floating-point numbers
complex64 the set of all complex numbers with float32 real and imaginary parts
complex128 the set of all complex numbers with float64 real and imaginary parts
byte alias for uint8
rune alias for int32

There is also a set of predeclared numeric types with implementation-specific sizes:

uint either 32 or 64 bits
int same size as uint
uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value

To avoid portability issues all numeric types are defined
types and thus distinct except
byte, which is an alias for uint8, and
rune, which is an alias for int32.
Conversions
are required when different numeric types are mixed in an expression
or assignment. For instance, int32 and int
are not the same type even though they may have the same size on a
particular architecture.

String types

A string type represents the set of string values.
A string value is a (possibly empty) sequence of bytes.
Strings are immutable: once created,
it is impossible to change the contents of a string.
The predeclared string type is string;
it is a defined type.

The length of a string s (its size in bytes) can be discovered using
the built-in function len.
The length is a compile-time constant if the string is a constant.
A string's bytes can be accessed by integer indices
0 through len(s)-1.
It is illegal to take the address of such an element; if
s[i] is the i'th byte of a
string, &s[i] is invalid.

Array types

An array is a numbered sequence of elements of a single
type, called the element type.
The number of elements is called the length and is never
negative.

The length is part of the array's type; it must evaluate to a
non-negative constantrepresentable by a value
of type int.
The length of array a can be discovered
using the built-in function len.
The elements can be addressed by integer indices
0 through len(a)-1.
Array types are always one-dimensional but may be composed to form
multi-dimensional types.

Slice types

A slice is a descriptor for a contiguous segment of an underlying array and
provides access to a numbered sequence of elements from that array.
A slice type denotes the set of all slices of arrays of its element type.
The value of an uninitialized slice is nil.

Like arrays, slices are indexable and have a length. The length of a
slice s can be discovered by the built-in function
len; unlike with arrays it may change during
execution. The elements can be addressed by integer indices
0 through len(s)-1. The slice index of a
given element may be less than the index of the same element in the
underlying array.

A slice, once initialized, is always associated with an underlying
array that holds its elements. A slice therefore shares storage
with its array and with other slices of the same array; by contrast,
distinct arrays always represent distinct storage.

The array underlying a slice may extend past the end of the slice.
The capacity is a measure of that extent: it is the sum of
the length of the slice and the length of the array beyond the slice;
a slice of length up to that capacity can be created by
slicing a new one from the original slice.
The capacity of a slice a can be discovered using the
built-in function cap(a).

A new, initialized slice value for a given element type T is
made using the built-in function
make,
which takes a slice type
and parameters specifying the length and optionally the capacity.
A slice created with make always allocates a new, hidden array
to which the returned slice value refers. That is, executing

make([]T, length, capacity)

produces the same slice as allocating an array and slicing
it, so these two expressions are equivalent:

make([]int, 50, 100)
new([100]int)[0:50]

Like arrays, slices are always one-dimensional but may be composed to construct
higher-dimensional objects.
With arrays of arrays, the inner arrays are, by construction, always the same length;
however with slices of slices (or arrays of slices), the inner lengths may vary dynamically.
Moreover, the inner slices must be initialized individually.

Struct types

A struct is a sequence of named elements, called fields, each of which has a
name and a type. Field names may be specified explicitly (IdentifierList) or
implicitly (EmbeddedField).
Within a struct, non-blank field names must
be unique.

A field declared with a type but no explicit field name is called an embedded field.
An embedded field must be specified as
a type name T or as a pointer to a non-interface type name *T,
and T itself may not be
a pointer type. The unqualified type name acts as the field name.

// A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
struct {
T1 // field name is T1
*T2 // field name is T2
P.T3 // field name is T3
*P.T4 // field name is T4
x, y int // field names are x and y
}

The following declaration is illegal because field names must be unique
in a struct type:

struct {
T // conflicts with embedded field *T and *P.T
*T // conflicts with embedded field T and *P.T
*P.T // conflicts with embedded field T and *T
}

A field or methodf of an
embedded field in a struct x is called promoted if
x.f is a legal selector that denotes
that field or method f.

Promoted fields act like ordinary fields
of a struct except that they cannot be used as field names in
composite literals of the struct.

Given a struct type S and a defined typeT, promoted methods are included in the method set of the struct as follows:

If S contains an embedded field T,
the method sets of S
and *S both include promoted methods with receiver
T. The method set of *S also
includes promoted methods with receiver *T.

If S contains an embedded field *T,
the method sets of S and *S both
include promoted methods with receiver T or
*T.

A field declaration may be followed by an optional string literal tag,
which becomes an attribute for all the fields in the corresponding
field declaration. An empty tag string is equivalent to an absent tag.
The tags are made visible through a reflection interface
and take part in type identity for structs
but are otherwise ignored.

Within a list of parameters or results, the names (IdentifierList)
must either all be present or all be absent. If present, each name
stands for one item (parameter or result) of the specified type and
all non-blank names in the signature
must be unique.
If absent, each type stands for one item of that type.
Parameter and result
lists are always parenthesized except that if there is exactly
one unnamed result it may be written as an unparenthesized type.

The final incoming parameter in a function signature may have
a type prefixed with ....
A function with such a parameter is called variadic and
may be invoked with zero or more arguments for that parameter.

Interface types

An interface type specifies a method set called its interface.
A variable of interface type can store a value of any type with a method set
that is any superset of the interface. Such a type is said to
implement the interface.
The value of an uninitialized variable of interface type is nil.

(where T stands for either S1 or S2)
then the File interface is implemented by both S1 and
S2, regardless of what other methods
S1 and S2 may have or share.

A type implements any interface comprising any subset of its methods
and may therefore implement several distinct interfaces. For
instance, all types implement the empty interface:

interface{}

Similarly, consider this interface specification,
which appears within a type declaration
to define an interface called Locker:

type Locker interface {
Lock()
Unlock()
}

If S1 and S2 also implement

func (p T) Lock() { … }
func (p T) Unlock() { … }

they implement the Locker interface as well
as the File interface.

An interface T may use a (possibly qualified) interface type
name E in place of a method specification. This is called
embedding interface E in T; it adds
all (exported and non-exported) methods of E to the interface
T.

The comparison operators== and != must be fully defined
for operands of the key type; thus the key type must not be a function, map, or
slice.
If the key type is an interface type, these
comparison operators must be defined for the dynamic key values;
failure will cause a run-time panic.

map[string]int
map[*T]struct{ x, y float64 }
map[string]interface{}

The number of map elements is called its length.
For a map m, it can be discovered using the
built-in function len
and may change during execution. Elements may be added during execution
using assignments and retrieved with
index expressions; they may be removed with the
delete built-in function.

A new, empty map value is made using the built-in
function make,
which takes the map type and an optional capacity hint as arguments:

make(map[string]int)
make(map[string]int, 100)

The initial capacity does not bound its size:
maps grow to accommodate the number of items
stored in them, with the exception of nil maps.
A nil map is equivalent to an empty map except that no elements
may be added.

The optional <- operator specifies the channel direction,
send or receive. If no direction is given, the channel is
bidirectional.
A channel may be constrained only to send or only to receive by
conversion or assignment.

chan T // can be used to send and receive values of type T
chan<- float64 // can only be used to send float64s
<-chan int // can only be used to receive ints

A new, initialized channel
value can be made using the built-in function
make,
which takes the channel type and an optional capacity as arguments:

make(chan int, 100)

The capacity, in number of elements, sets the size of the buffer in the channel.
If the capacity is zero or absent, the channel is unbuffered and communication
succeeds only when both a sender and receiver are ready. Otherwise, the channel
is buffered and communication succeeds without blocking if the buffer
is not full (sends) or not empty (receives).
A nil channel is never ready for communication.

A channel may be closed with the built-in function
close.
The multi-valued assignment form of the
receive operator
reports whether a received value was sent before
the channel was closed.

A single channel may be used in
send statements,
receive operations,
and calls to the built-in functions
cap and
len
by any number of goroutines without further synchronization.
Channels act as first-in-first-out queues.
For example, if one goroutine sends values on a channel
and a second goroutine receives them, the values are
received in the order sent.

Properties of types and values

Type identity

Two types are either identical or different.

A defined type is always different from any other type.
Otherwise, two types are identical if their underlying type literals are
structurally equivalent; that is, they have the same literal structure and corresponding
components have identical types. In detail:

Two array types are identical if they have identical element types and
the same array length.

Two slice types are identical if they have identical element types.

Two struct types are identical if they have the same sequence of fields,
and if corresponding fields have the same names, and identical types,
and identical tags.
Non-exported field names from different
packages are always different.

Two pointer types are identical if they have identical base types.

Two function types are identical if they have the same number of parameters
and result values, corresponding parameter and result types are
identical, and either both functions are variadic or neither is.
Parameter and result names are not required to match.

Two interface types are identical if they have the same set of methods
with the same names and identical function types.
Non-exported method names from different
packages are always different. The order of the methods is irrelevant.

Two map types are identical if they have identical key and element types.

Two channel types are identical if they have identical element types and
the same direction.

B0 and B1 are different because they are new types
created by distinct type definitions;
func(int, float64) *B0 and func(x int, y float64) *[]string
are different because B0 is different from []string.

Assignability

A value x is assignable to a variable of type T
("x is assignable to T") if one of the following conditions applies:

Representability

T is a floating-point type and x can be rounded to T's
precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE
negative zero further simplified to an unsigned zero. Note that constant values never result
in an IEEE negative zero, NaN, or infinity.

T is a complex type, and x's
componentsreal(x) and imag(x)
are representable by values of T's component type (float32 or
float64).

x T x is representable by a value of T because
'a' byte 97 is in the set of byte values
97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers
"foo" string "foo" is in the set of string values
1024 int16 1024 is in the set of 16-bit integers
42.0 byte 42 is in the set of unsigned 8-bit integers
1e10 uint64 10000000000 is in the set of unsigned 64-bit integers
2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values
-1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0
0i int 0 is an integer value
(42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values

x T x is not representable by a value of T because
0 bool 0 is not in the set of boolean values
'a' string 'a' is a rune, it is not in the set of string values
1024 byte 1024 is not in the set of unsigned 8-bit integers
-1 uint16 -1 is not in the set of unsigned 16-bit integers
1.1 int 1.1 is not an integer value
42i float32 (0 + 42i) is not in the set of float32 values
1e1000 float64 1e1000 overflows to IEEE +Inf after rounding

Blocks

A block is a possibly empty sequence of declarations and statements
within matching brace brackets.

Declarations and scope

A declaration binds a non-blank identifier to a
constant,
type,
variable,
function,
label, or
package.
Every identifier in a program must be declared.
No identifier may be declared twice in the same block, and
no identifier may be declared in both the file and package block.

The blank identifier may be used like any other identifier
in a declaration, but it does not introduce a binding and thus is not declared.
In the package block, the identifier init may only be used for
init function declarations,
and like the blank identifier it does not introduce a new binding.

The scope of an identifier denoting a constant, type, variable,
or function (but not method) declared at top level (outside any
function) is the package block.

The scope of the package name of an imported package is the file block
of the file containing the import declaration.

The scope of an identifier denoting a method receiver, function parameter,
or result variable is the function body.

The scope of a constant or variable identifier declared
inside a function begins at the end of the ConstSpec or VarSpec
(ShortVarDecl for short variable declarations)
and ends at the end of the innermost containing block.

The scope of a type identifier declared inside a function
begins at the identifier in the TypeSpec
and ends at the end of the innermost containing block.

An identifier declared in a block may be redeclared in an inner block.
While the identifier of the inner declaration is in scope, it denotes
the entity declared by the inner declaration.

The package clause is not a declaration; the package name
does not appear in any scope. Its purpose is to identify the files belonging
to the same package and to specify the default package name for import
declarations.

Label scopes

Labels are declared by labeled statements and are
used in the "break",
"continue", and
"goto" statements.
It is illegal to define a label that is never used.
In contrast to other identifiers, labels are not block scoped and do
not conflict with identifiers that are not labels. The scope of a label
is the body of the function in which it is declared and excludes
the body of any nested function.

Blank identifier

The blank identifier is represented by the underscore character _.
It serves as an anonymous placeholder instead of a regular (non-blank)
identifier and has special meaning in declarations,
as an operand, and in assignments.

Predeclared identifiers

The following identifiers are implicitly declared in the
universe block:

Uniqueness of identifiers

Given a set of identifiers, an identifier is called unique if it is
different from every other in the set.
Two identifiers are different if they are spelled differently, or if they
appear in different packages and are not
exported. Otherwise, they are the same.

Constant declarations

A constant declaration binds a list of identifiers (the names of
the constants) to the values of a list of constant expressions.
The number of identifiers must be equal
to the number of expressions, and the nth identifier on
the left is bound to the value of the nth expression on the
right.

If the type is present, all constants take the type specified, and
the expressions must be assignable to that type.
If the type is omitted, the constants take the
individual types of the corresponding expressions.
If the expression values are untyped constants,
the declared constants remain untyped and the constant identifiers
denote the constant values. For instance, if the expression is a
floating-point literal, the constant identifier denotes a floating-point
constant, even if the literal's fractional part is zero.

Within a parenthesized const declaration list the
expression list may be omitted from any but the first ConstSpec.
Such an empty list is equivalent to the textual substitution of the
first preceding non-empty expression list and its type if any.
Omitting the list of expressions is therefore equivalent to
repeating the previous list. The number of identifiers must be equal
to the number of expressions in the previous list.
Together with the iota constant generator
this mechanism permits light-weight declaration of sequential values:

Iota

Within a constant declaration, the predeclared identifier
iota represents successive untyped integer
constants. Its value is the index of the respective ConstSpec
in that constant declaration, starting at zero.
It can be used to construct a set of related constants:

A defined type may have methods associated with it.
It does not inherit any methods bound to the given type,
but the method set
of an interface type or of elements of a composite type remains unchanged:

// A Mutex is a data type with two methods, Lock and Unlock.
type Mutex struct { /* Mutex fields */ }
func (m *Mutex) Lock() { /* Lock implementation */ }
func (m *Mutex) Unlock() { /* Unlock implementation */ }
// NewMutex has the same composition as Mutex but its method set is empty.
type NewMutex Mutex
// The method set of PtrMutex's underlying type *Mutex remains unchanged,
// but the method set of PtrMutex is empty.
type PtrMutex *Mutex
// The method set of *PrintableMutex contains the methods
// Lock and Unlock bound to its embedded field Mutex.
type PrintableMutex struct {
Mutex
}
// MyBlock is an interface type that has the same method set as Block.
type MyBlock Block

Type definitions may be used to define different boolean, numeric,
or string types and associate methods with them:

If a list of expressions is given, the variables are initialized
with the expressions following the rules for assignments.
Otherwise, each variable is initialized to its zero value.

If a type is present, each variable is given that type.
Otherwise, each variable is given the type of the corresponding
initialization value in the assignment.
If that value is an untyped constant, it is first
converted to its default type;
if it is an untyped boolean value, it is first converted to type bool.
The predeclared value nil cannot be used to initialize a variable
with no explicit type.

Unlike regular variable declarations, a short variable declaration may redeclare
variables provided they were originally declared earlier in the same block
(or the parameter lists if the block is the function body) with the same type,
and at least one of the non-blank variables is new.
As a consequence, redeclaration can only appear in a multi-variable short declaration.
Redeclaration does not introduce a new variable; it just assigns a new value to the original.

The receiver is specified via an extra parameter section preceding the method
name. That parameter section must declare a single non-variadic parameter, the receiver.
Its type must be of the form T or *T (possibly using
parentheses) where T is a type name. The type denoted by T is called
the receiver base type; it must not be a pointer or interface type and
it must be defined in the same package as the method.
The method is said to be bound to the base type and the method name
is visible only within selectors for type T
or *T.

A non-blank receiver identifier must be
unique in the method signature.
If the receiver's value is not referenced inside the body of the method,
its identifier may be omitted in the declaration. The same applies in
general to parameters of functions and methods.

For a base type, the non-blank names of methods bound to it must be unique.
If the base type is a struct type,
the non-blank method and field names must be distinct.

bind the methods Length and Scale,
with receiver type *Point,
to the base type Point.

The type of a method is the type of a function with the receiver as first
argument. For instance, the method Scale has type

func(p *Point, factor float64)

However, a function declared this way is not a method.

Expressions

An expression specifies the computation of a value by applying
operators and functions to operands.

Operands

Operands denote the elementary values in an expression. An operand may be a
literal, a (possibly qualified)
non-blank identifier denoting a
constant,
variable, or
function,
or a parenthesized expression.

Qualified identifiers

A qualified identifier accesses an identifier in a different package, which
must be imported.
The identifier must be exported and
declared in the package block of that package.

math.Sin // denotes the Sin function in package math

Composite literals

Composite literals construct values for structs, arrays, slices, and maps
and create a new value each time they are evaluated.
They consist of the type of the literal followed by a brace-bound list of elements.
Each element may optionally be preceded by a corresponding key.

The LiteralType's underlying type must be a struct, array, slice, or map type
(the grammar enforces this constraint except when the type is given
as a TypeName).
The types of the elements and keys must be assignable
to the respective field, element, and key types of the literal type;
there is no additional conversion.
The key is interpreted as a field name for struct literals,
an index for array and slice literals, and a key for map literals.
For map literals, all elements must have a key. It is an error
to specify multiple elements with the same field name or
constant key value. For non-constant map keys, see the section on
evaluation order.

For struct literals the following rules apply:

A key must be a field name declared in the struct type.

An element list that does not contain any keys must
list an element for each struct field in the
order in which the fields are declared.

If any element has a key, every element must have a key.

An element list that contains keys does not need to
have an element for each struct field. Omitted fields
get the zero value for that field.

A literal may omit the element list; such a literal evaluates
to the zero value for its type.

It is an error to specify an element for a non-exported
field of a struct belonging to a different package.

The length of an array literal is the length specified in the literal type.
If fewer elements than the length are provided in the literal, the missing
elements are set to the zero value for the array element type.
It is an error to provide elements with index values outside the index range
of the array. The notation ... specifies an array length equal
to the maximum element index plus one.

A slice literal describes the entire underlying array literal.
Thus the length and capacity of a slice literal are the maximum
element index plus one. A slice literal has the form

[]T{x1, x2, … xn}

and is shorthand for a slice operation applied to an array:

tmp := [n]T{x1, x2, … xn}
tmp[0 : n]

Within a composite literal of array, slice, or map type T,
elements or map keys that are themselves composite literals may elide the respective
literal type if it is identical to the element or key type of T.
Similarly, elements or keys that are addresses of composite literals may elide
the &T when the element or key type is *T.

A parsing ambiguity arises when a composite literal using the
TypeName form of the LiteralType appears as an operand between the
keyword and the opening brace of the block
of an "if", "for", or "switch" statement, and the composite literal
is not enclosed in parentheses, square brackets, or curly braces.
In this rare case, the opening brace of the literal is erroneously parsed
as the one introducing the block of statements. To resolve the ambiguity,
the composite literal must appear within parentheses.

Function literals are closures: they may refer to variables
defined in a surrounding function. Those variables are then shared between
the surrounding function and the function literal, and they survive as long
as they are accessible.

Primary expressions

Primary expressions are the operands for unary and binary expressions.

Selectors

denotes the field or method f of the value x
(or sometimes *x; see below).
The identifier f is called the (field or method) selector;
it must not be the blank identifier.
The type of the selector expression is the type of f.
If x is a package name, see the section on
qualified identifiers.

A selector f may denote a field or method f of
a type T, or it may refer
to a field or method f of a nested
embedded field of T.
The number of embedded fields traversed
to reach f is called its depth in T.
The depth of a field or method f
declared in T is zero.
The depth of a field or method f declared in
an embedded field A in T is the
depth of f in A plus one.

The following rules apply to selectors:

For a value x of type T or *T
where T is not a pointer or interface type,
x.f denotes the field or method at the shallowest depth
in T where there
is such an f.
If there is not exactly one f
with shallowest depth, the selector expression is illegal.

For a value x of type I where I
is an interface type, x.f denotes the actual method with name
f of the dynamic value of x.
If there is no method with name f in the
method set of I, the selector
expression is illegal.

As an exception, if the type of x is a defined
pointer type and (*x).f is a valid selector expression denoting a field
(but not a method), x.f is shorthand for (*x).f.

In all other cases, x.f is illegal.

If x is of pointer type and has the value
nil and x.f denotes a struct field,
assigning to or evaluating x.f
causes a run-time panic.

For a method with a value receiver, one can derive a function
with an explicit pointer receiver, so

(*T).Mv

yields a function value representing Mv with signature

func(tv *T, a int) int

Such a function indirects through the receiver to create a value
to pass as the receiver to the underlying method;
the method does not overwrite the value whose address is passed in
the function call.

The final case, a value-receiver function for a pointer-receiver method,
is illegal because pointer-receiver methods are not in the method set
of the value type.

Function values derived from methods are called with function call syntax;
the receiver is provided as the first argument to the call.
That is, given f := T.Mv, f is invoked
as f(t, 7) not t.f(7).
To construct a function that binds the receiver, use a
function literal or
method value.

It is legal to derive a function value from a method of an interface type.
The resulting function takes an explicit receiver of that interface type.

Method values

If the expression x has static type T and
M is in the method set of type T,
x.M is called a method value.
The method value x.M is a function value that is callable
with the same arguments as a method call of x.M.
The expression x is evaluated and saved during the evaluation of the
method value; the saved copy is then used as the receiver in any calls,
which may be executed later.

The type T may be an interface or non-interface type.

As in the discussion of method expressions above,
consider a struct type T with two methods,
Mv, whose receiver is of type T, and
Mp, whose receiver is of type *T.

Slice expressions

Slice expressions construct a substring or slice from a string, array, pointer
to array, or slice. There are two variants: a simple form that specifies a low
and high bound, and a full form that also specifies a bound on the capacity.

Simple slice expressions

For a string, array, pointer to array, or slice a, the primary expression

a[low : high]

constructs a substring or slice. The indiceslow and
high select which elements of operand a appear
in the result. The result has indices starting at 0 and length equal to
high - low.
After slicing the array a

a := [5]int{1, 2, 3, 4, 5}
s := a[1:4]

the slice s has type []int, length 3, capacity 4, and elements

s[0] == 2
s[1] == 3
s[2] == 4

For convenience, any of the indices may be omitted. A missing low
index defaults to zero; a missing high index defaults to the length of the
sliced operand:

If a is a pointer to an array, a[low : high] is shorthand for
(*a)[low : high].

For arrays or strings, the indices are in range if
0 <= low <= high <= len(a),
otherwise they are out of range.
For slices, the upper index bound is the slice capacity cap(a) rather than the length.
A constant index must be non-negative and
representable by a value of type
int; for arrays or constant strings, constant indices must also be in range.
If both indices are constant, they must satisfy low <= high.
If the indices are out of range at run time, a run-time panic occurs.

Except for untyped strings, if the sliced operand is a string or slice,
the result of the slice operation is a non-constant value of the same type as the operand.
For untyped string operands the result is a non-constant value of type string.
If the sliced operand is an array, it must be addressable
and the result of the slice operation is a slice with the same element type as the array.

If the sliced operand of a valid slice expression is a nil slice, the result
is a nil slice. Otherwise, if the result is a slice, it shares its underlying
array with the operand.

Full slice expressions

For an array, pointer to array, or slice a (but not a string), the primary expression

a[low : high : max]

constructs a slice of the same type, and with the same length and elements as the simple slice
expression a[low : high]. Additionally, it controls the resulting slice's capacity
by setting it to max - low. Only the first index may be omitted; it defaults to 0.
After slicing the array a

a := [5]int{1, 2, 3, 4, 5}
t := a[1:3:5]

the slice t has type []int, length 2, capacity 4, and elements

t[0] == 2
t[1] == 3

As for simple slice expressions, if a is a pointer to an array,
a[low : high : max] is shorthand for (*a)[low : high : max].
If the sliced operand is an array, it must be addressable.

The indices are in range if 0 <= low <= high <= max <= cap(a),
otherwise they are out of range.
A constant index must be non-negative and
representable by a value of type
int; for arrays, constant indices must also be in range.
If multiple indices are constant, the constants that are present must be in range relative to each
other.
If the indices are out of range at run time, a run-time panic occurs.

Type assertions

For an expression x of interface type
and a type T, the primary expression

x.(T)

asserts that x is not nil
and that the value stored in x is of type T.
The notation x.(T) is called a type assertion.

More precisely, if T is not an interface type, x.(T) asserts
that the dynamic type of x is identical
to the type T.
In this case, T must implement the (interface) type of x;
otherwise the type assertion is invalid since it is not possible for x
to store a value of type T.
If T is an interface type, x.(T) asserts that the dynamic type
of x implements the interface T.

If the type assertion holds, the value of the expression is the value
stored in x and its type is T. If the type assertion is false,
a run-time panic occurs.
In other words, even though the dynamic type of x
is known only at run time, the type of x.(T) is
known to be T in a correct program.

A type assertion used in an assignment or initialization of the special form

v, ok = x.(T)
v, ok := x.(T)
var v, ok = x.(T)
var v, ok T1 = x.(T)

yields an additional untyped boolean value. The value of ok is true
if the assertion holds. Otherwise it is false and the value of v is
the zero value for type T.
No run-time panic occurs in this case.

Calls

Given an expression f of function type
F,

f(a1, a2, … an)

calls f with arguments a1, a2, … an.
Except for one special case, arguments must be single-valued expressions
assignable to the parameter types of
F and are evaluated before the function is called.
The type of the expression is the result type
of F.
A method invocation is similar but the method itself
is specified as a selector upon a value of the receiver type for
the method.

In a function call, the function value and arguments are evaluated in
the usual order.
After they are evaluated, the parameters of the call are passed by value to the function
and the called function begins execution.
The return parameters of the function are passed by value
back to the calling function when the function returns.

As a special case, if the return values of a function or method
g are equal in number and individually
assignable to the parameters of another function or method
f, then the call f(g(parameters_of_g))
will invoke f after binding the return values of
g to the parameters of f in order. The call
of f must contain no parameters other than the call of g,
and g must have at least one return value.
If f has a final ... parameter, it is
assigned the return values of g that remain after
assignment of regular parameters.

A method call x.m() is valid if the method set
of (the type of) x contains m and the
argument list can be assigned to the parameter list of m.
If x is addressable and &x's method
set contains m, x.m() is shorthand
for (&x).m():

var p Point
p.Scale(3.5)

There is no distinct method type and there are no method literals.

Passing arguments to ... parameters

If f is variadic with a final
parameter p of type ...T, then within f
the type of p is equivalent to type []T.
If f is invoked with no actual arguments for p,
the value passed to p is nil.
Otherwise, the value passed is a new slice
of type []T with a new underlying array whose successive elements
are the actual arguments, which all must be assignable
to T. The length and capacity of the slice is therefore
the number of arguments bound to p and may differ for each
call site.

Comparisons are discussed elsewhere.
For other binary operators, the operand types must be identical
unless the operation involves shifts or untyped constants.
For operations involving constants only, see the section on
constant expressions.

Except for shift operations, if one operand is an untyped constant
and the other operand is not, the constant is converted
to the type of the other operand.

The right operand in a shift expression must have unsigned integer type
or be an untyped constant representable by a
value of type uint.
If the left operand of a non-constant shift expression is an untyped constant,
it is first converted to the type it would assume if the shift expression were
replaced by its left operand alone.

Operator precedence

Unary operators have the highest precedence.
As the ++ and -- operators form
statements, not expressions, they fall
outside the operator hierarchy.
As a consequence, statement *p++ is the same as (*p)++.

Binary operators of the same precedence associate from left to right.
For instance, x / y * z is the same as (x / y) * z.

+x
23 + 3*x[i]
x <= f()
^a >> b
f() || g()
x == y+1 && <-chanPtr > 0

Arithmetic operators

Arithmetic operators apply to numeric values and yield a result of the same
type as the first operand. The four standard arithmetic operators (+,
-, *, /) apply to integer,
floating-point, and complex types; + also applies to strings.
The bitwise logical and shift operators apply to integers only.

If the divisor is a constant, it must not be zero.
If the divisor is zero at run time, a run-time panic occurs.
If the dividend is non-negative and the divisor is a constant power of 2,
the division may be replaced by a right shift, and computing the remainder may
be replaced by a bitwise AND operation:

x x / 4 x % 4 x >> 2 x & 3
11 2 3 2 3
-11 -2 -3 -3 1

The shift operators shift the left operand by the shift count specified by the
right operand. They implement arithmetic shifts if the left operand is a signed
integer and logical shifts if it is an unsigned integer.
There is no upper limit on the shift count. Shifts behave
as if the left operand is shifted n times by 1 for a shift
count of n.
As a result, x << 1 is the same as x*2
and x >> 1 is the same as
x/2 but truncated towards negative infinity.

For integer operands, the unary operators
+, -, and ^ are defined as
follows:

Integer overflow

For unsigned integer values, the operations +,
-, *, and << are
computed modulo 2n, where n is the bit width of
the unsigned integer's type.
Loosely speaking, these unsigned integer operations
discard high bits upon overflow, and programs may rely on "wrap around".

For signed integers, the operations +,
-, *, /, and << may legally
overflow and the resulting value exists and is deterministically defined
by the signed integer representation, the operation, and its operands.
No exception is raised as a result of overflow.
A compiler may not optimize code under the assumption that overflow does
not occur. For instance, it may not assume that x < x + 1 is always true.

Floating-point operators

For floating-point and complex numbers,
+x is the same as x,
while -x is the negation of x.
The result of a floating-point or complex division by zero is not specified beyond the
IEEE-754 standard; whether a run-time panic
occurs is implementation-specific.

An implementation may combine multiple floating-point operations into a single
fused operation, possibly across statements, and produce a result that differs
from the value obtained by executing and rounding the instructions individually.
A floating-point type conversion explicitly rounds to
the precision of the target type, preventing fusion that would discard that rounding.

For instance, some architectures provide a "fused multiply and add" (FMA) instruction
that computes x*y + z without rounding the intermediate result x*y.
These examples show when a Go implementation can use that instruction:

Comparison operators

In any comparison, the first operand
must be assignable
to the type of the second operand, or vice versa.

The equality operators == and != apply
to operands that are comparable.
The ordering operators <, <=, >, and >=
apply to operands that are ordered.
These terms and the result of the comparisons are defined as follows:

Boolean values are comparable.
Two boolean values are equal if they are either both
true or both false.

Integer values are comparable and ordered, in the usual way.

Floating-point values are comparable and ordered,
as defined by the IEEE-754 standard.

Complex values are comparable.
Two complex values u and v are
equal if both real(u) == real(v) and
imag(u) == imag(v).

String values are comparable and ordered, lexically byte-wise.

Pointer values are comparable.
Two pointer values are equal if they point to the same variable or if both have value nil.
Pointers to distinct zero-size variables may or may not be equal.

Channel values are comparable.
Two channel values are equal if they were created by the same call to
make
or if both have value nil.

Interface values are comparable.
Two interface values are equal if they have identical dynamic types
and equal dynamic values or if both have value nil.

A value x of non-interface type X and
a value t of interface type T are comparable when values
of type X are comparable and
X implements T.
They are equal if t's dynamic type is identical to X
and t's dynamic value is equal to x.

Struct values are comparable if all their fields are comparable.
Two struct values are equal if their corresponding
non-blank fields are equal.

Array values are comparable if values of the array element type are comparable.
Two array values are equal if their corresponding elements are equal.

A comparison of two interface values with identical dynamic types
causes a run-time panic if values
of that type are not comparable. This behavior applies not only to direct interface
value comparisons but also when comparing arrays of interface values
or structs with interface-valued fields.

Slice, map, and function values are not comparable.
However, as a special case, a slice, map, or function value may
be compared to the predeclared identifier nil.
Comparison of pointer, channel, and interface values to nil
is also allowed and follows from the general rules above.

Address operators

For an operand x of type T, the address operation
&x generates a pointer of type *T to x.
The operand must be addressable,
that is, either a variable, pointer indirection, or slice indexing
operation; or a field selector of an addressable struct operand;
or an array indexing operation of an addressable array.
As an exception to the addressability requirement, x may also be a
(possibly parenthesized)
composite literal.
If the evaluation of x would cause a run-time panic,
then the evaluation of &x does too.

For an operand x of pointer type *T, the pointer
indirection *x denotes the variable of type T pointed
to by x.
If x is nil, an attempt to evaluate *x
will cause a run-time panic.

Receive operator

For an operand ch of channel type,
the value of the receive operation <-ch is the value received
from the channel ch. The channel direction must permit receive operations,
and the type of the receive operation is the element type of the channel.
The expression blocks until a value is available.
Receiving from a nil channel blocks forever.
A receive operation on a closed channel can always proceed
immediately, yielding the element type's zero value
after any previously sent values have been received.

A receive expression used in an assignment or initialization of the special form

x, ok = <-ch
x, ok := <-ch
var x, ok = <-ch
var x, ok T = <-ch

yields an additional untyped boolean result reporting whether the
communication succeeded. The value of ok is true
if the value received was delivered by a successful send operation to the
channel, or false if it is a zero value generated because the
channel is closed and empty.

Conversions

Conversions are expressions of the form T(x)
where T is a type and x is an expression
that can be converted to type T.

Specific rules apply to (non-constant) conversions between numeric types or
to and from a string type.
These conversions may change the representation of x
and incur a run-time cost.
All other conversions only change the type but not the representation
of x.

There is no linguistic mechanism to convert between pointers and integers.
The package unsafe
implements this functionality under
restricted circumstances.

Conversions between numeric types

For the conversion of non-constant numeric values, the following rules apply:

When converting between integer types, if the value is a signed integer, it is
sign extended to implicit infinite precision; otherwise it is zero extended.
It is then truncated to fit in the result type's size.
For example, if v := uint16(0x10F0), then uint32(int8(v)) == 0xFFFFFFF0.
The conversion always yields a valid value; there is no indication of overflow.

When converting a floating-point number to an integer, the fraction is discarded
(truncation towards zero).

When converting an integer or floating-point number to a floating-point type,
or a complex number to another complex type, the result value is rounded
to the precision specified by the destination type.
For instance, the value of a variable x of type float32
may be stored using additional precision beyond that of an IEEE-754 32-bit number,
but float32(x) represents the result of rounding x's value to
32-bit precision. Similarly, x + 0.1 may use more than 32 bits
of precision, but float32(x + 0.1) does not.

In all non-constant conversions involving floating-point or complex values,
if the result type cannot represent the value the conversion
succeeds but the result value is implementation-dependent.

Conversions to and from a string type

Converting a signed or unsigned integer value to a string type yields a
string containing the UTF-8 representation of the integer. Values outside
the range of valid Unicode code points are converted to "\uFFFD".

Constant expressions

Constant expressions may contain only constant
operands and are evaluated at compile time.

Untyped boolean, numeric, and string constants may be used as operands
wherever it is legal to use an operand of boolean, numeric, or string type,
respectively.

A constant comparison always yields
an untyped boolean constant. If the left operand of a constant
shift expression is an untyped constant, the
result is an integer constant; otherwise it is a constant of the same
type as the left operand, which must be of
integer type.

Any other operation on untyped constants results in an untyped constant of the
same kind; that is, a boolean, integer, floating-point, complex, or string
constant.
If the untyped operands of a binary operation (other than a shift) are of
different kinds, the result is of the operand's kind that appears later in this
list: integer, rune, floating-point, complex.
For example, an untyped integer constant divided by an
untyped complex constant yields an untyped complex constant.

Constant expressions are always evaluated exactly; intermediate values and the
constants themselves may require precision significantly larger than supported
by any predeclared type in the language. The following are legal declarations:

The divisor of a constant division or remainder operation must not be zero:

3.14 / 0.0 // illegal: division by zero

The values of typed constants must always be accurately
representable by values
of the constant type. The following constant expressions are illegal:

uint(-1) // -1 cannot be represented as a uint
int(3.14) // 3.14 cannot be represented as an int
int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64
Four * 300 // operand 300 cannot be represented as an int8 (type of Four)
Four * 100 // product 400 cannot be represented as an int8 (type of Four)

The mask used by the unary bitwise complement operator ^ matches
the rule for non-constants: the mask is all 1s for unsigned constants
and -1 for signed and untyped constants.

Implementation restriction: A compiler may use rounding while
computing untyped floating-point or complex constant expressions; see
the implementation restriction in the section
on constants. This rounding may cause a
floating-point constant expression to be invalid in an integer
context, even if it would be integral when calculated using infinite
precision, and vice versa.

the function calls and communication happen in the order
f(), h(), i(), j(),
<-c, g(), and k().
However, the order of those events compared to the evaluation
and indexing of x and the evaluation
of y is not specified.

a := 1
f := func() int { a++; return a }
x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified
m := map[int]int{a: 1, a: 2} // m may be {2: 1} or {2: 2}: evaluation order between the two map assignments is not specified
n := map[int]int{a: f()} // n may be {2: 3} or {3: 3}: evaluation order between the key and the value is not specified

At package level, initialization dependencies override the left-to-right rule
for individual initialization expressions, but not for operands within each
expression:

The function calls happen in the order
u(), sqr(), v(),
f(), v(), and g().

Floating-point operations within a single expression are evaluated according to
the associativity of the operators. Explicit parentheses affect the evaluation
by overriding the default associativity.
In the expression x + (y + z) the addition y + z
is performed before adding x.

Send statements

A send statement sends a value on a channel.
The channel expression must be of channel type,
the channel direction must permit send operations,
and the type of the value to be sent must be assignable
to the channel's element type.

Both the channel and the value expression are evaluated before communication
begins. Communication blocks until the send can proceed.
A send on an unbuffered channel can proceed if a receiver is ready.
A send on a buffered channel can proceed if there is room in the buffer.
A send on a closed channel proceeds by causing a run-time panic.
A send on a nil channel blocks forever.

ch <- 3 // send value 3 to channel ch

IncDec statements

The "++" and "--" statements increment or decrement their operands
by the untyped constant1.
As with an assignment, the operand must be addressable
or a map index expression.

Assignments

Each left-hand side operand must be addressable,
a map index expression, or (for = assignments only) the
blank identifier.
Operands may be parenthesized.

x = 1
*p = f()
a[i] = 23
(k) = <-ch // same as: k = <-ch

An assignment operationxop=y where op is a binary arithmetic operator
is equivalent to x=xop(y) but evaluates x
only once. The op= construct is a single token.
In assignment operations, both the left- and right-hand expression lists
must contain exactly one single-valued expression, and the left-hand
expression must not be the blank identifier.

a[i] <<= 2
i &^= 1<<n

A tuple assignment assigns the individual elements of a multi-valued
operation to a list of variables. There are two forms. In the
first, the right hand operand is a single multi-valued expression
such as a function call, a channel or
map operation, or a type assertion.
The number of operands on the left
hand side must match the number of values. For instance, if
f is a function returning two values,

x, y = f()

assigns the first value to x and the second to y.
In the second form, the number of operands on the left must equal the number
of expressions on the right, each of which must be single-valued, and the
nth expression on the right is assigned to the nth
operand on the left:

one, two, three = '一', '二', '三'

The blank identifier provides a way to
ignore right-hand side values in an assignment:

In assignments, each value must be assignable
to the type of the operand to which it is assigned, with the following special cases:

Any typed value may be assigned to the blank identifier.

If an untyped constant
is assigned to a variable of interface type or the blank identifier,
the constant is first converted to its
default type.

If an untyped boolean value is assigned to a variable of interface type or
the blank identifier, it is first converted to type bool.

If statements

"If" statements specify the conditional execution of two branches
according to the value of a boolean expression. If the expression
evaluates to true, the "if" branch is executed, otherwise, if
present, the "else" branch is executed.

There are two forms: expression switches and type switches.
In an expression switch, the cases contain expressions that are compared
against the value of the switch expression.
In a type switch, the cases contain types that are compared against the
type of a specially annotated switch expression.
The switch expression is evaluated exactly once in a switch statement.

Expression switches

In an expression switch,
the switch expression is evaluated and
the case expressions, which need not be constants,
are evaluated left-to-right and top-to-bottom; the first one that equals the
switch expression
triggers execution of the statements of the associated case;
the other cases are skipped.
If no case matches and there is a "default" case,
its statements are executed.
There can be at most one default case and it may appear anywhere in the
"switch" statement.
A missing switch expression is equivalent to the boolean value
true.

If the switch expression evaluates to an untyped constant, it is first
converted to its default type;
if it is an untyped boolean value, it is first converted to type bool.
The predeclared untyped value nil cannot be used as a switch expression.

If a case expression is untyped, it is first converted
to the type of the switch expression.
For each (possibly converted) case expression x and the value t
of the switch expression, x == t must be a valid comparison.

In other words, the switch expression is treated as if it were used to declare and
initialize a temporary variable t without explicit type; it is that
value of t against which each case expression x is tested
for equality.

In a case or default clause, the last non-empty statement
may be a (possibly labeled)
"fallthrough" statement to
indicate that control should flow from the end of this clause to
the first statement of the next clause.
Otherwise control flows to the end of the "switch" statement.
A "fallthrough" statement may appear as the last statement of all
but the last clause of an expression switch.

The switch expression may be preceded by a simple statement, which
executes before the expression is evaluated.

Implementation restriction: A compiler may disallow multiple case
expressions evaluating to the same constant.
For instance, the current compilers disallow duplicate integer,
floating point, or string constants in case expressions.

Type switches

A type switch compares types rather than values. It is otherwise similar
to an expression switch. It is marked by a special switch expression that
has the form of a type assertion
using the reserved word type rather than an actual type:

switch x.(type) {
// cases
}

Cases then match actual types T against the dynamic type of the
expression x. As with type assertions, x must be of
interface type, and each non-interface type
T listed in a case must implement the type of x.
The types listed in the cases of a type switch must all be
different.

The TypeSwitchGuard may include a
short variable declaration.
When that form is used, the variable is declared at the end of the
TypeSwitchCase in the implicit block of each clause.
In clauses with a case listing exactly one type, the variable
has that type; otherwise, the variable has the type of the expression
in the TypeSwitchGuard.

Instead of a type, a case may use the predeclared identifier
nil;
that case is selected when the expression in the TypeSwitchGuard
is a nil interface value.
There may be at most one nil case.

Given an expression x of type interface{},
the following type switch:

switch i := x.(type) {
case nil:
printString("x is nil") // type of i is type of x (interface{})
case int:
printInt(i) // type of i is int
case float64:
printFloat64(i) // type of i is float64
case func(int) float64:
printFunction(i) // type of i is func(int) float64
case bool, string:
printString("type is bool or string") // type of i is type of x (interface{})
default:
printString("don't know the type") // type of i is type of x (interface{})
}

For statements with single condition

In its simplest form, a "for" statement specifies the repeated execution of
a block as long as a boolean condition evaluates to true.
The condition is evaluated before each iteration.
If the condition is absent, it is equivalent to the boolean value
true.

for a < b {
a *= 2
}

For statements with for clause

A "for" statement with a ForClause is also controlled by its condition, but
additionally it may specify an init
and a post statement, such as an assignment,
an increment or decrement statement. The init statement may be a
short variable declaration, but the post statement must not.
Variables declared by the init statement are re-used in each iteration.

If non-empty, the init statement is executed once before evaluating the
condition for the first iteration;
the post statement is executed after each execution of the block (and
only if the block was executed).
Any element of the ForClause may be empty but the
semicolons are
required unless there is only a condition.
If the condition is absent, it is equivalent to the boolean value
true.

for cond { S() } is the same as for ; cond ; { S() }
for { S() } is the same as for true { S() }

For statements with range clause

A "for" statement with a "range" clause
iterates through all entries of an array, slice, string or map,
or values received on a channel. For each entry it assigns iteration values
to corresponding iteration variables if present and then executes the block.

The expression on the right in the "range" clause is called the range expression,
which may be an array, pointer to an array, slice, string, map, or channel permitting
receive operations.
As with an assignment, if present the operands on the left must be
addressable or map index expressions; they
denote the iteration variables. If the range expression is a channel, at most
one iteration variable is permitted, otherwise there may be up to two.
If the last iteration variable is the blank identifier,
the range clause is equivalent to the same clause without that identifier.

The range expression x is evaluated once before beginning the loop,
with one exception: if at most one iteration variable is present and
len(x) is constant,
the range expression is not evaluated.

Function calls on the left are evaluated once per iteration.
For each iteration, iteration values are produced as follows
if the respective iteration variables are present:

For an array, pointer to array, or slice value a, the index iteration
values are produced in increasing order, starting at element index 0.
If at most one iteration variable is present, the range loop produces
iteration values from 0 up to len(a)-1 and does not index into the array
or slice itself. For a nil slice, the number of iterations is 0.

For a string value, the "range" clause iterates over the Unicode code points
in the string starting at byte index 0. On successive iterations, the index value will be the
index of the first byte of successive UTF-8-encoded code points in the string,
and the second value, of type rune, will be the value of
the corresponding code point. If the iteration encounters an invalid
UTF-8 sequence, the second value will be 0xFFFD,
the Unicode replacement character, and the next iteration will advance
a single byte in the string.

The iteration order over maps is not specified
and is not guaranteed to be the same from one iteration to the next.
If a map entry that has not yet been reached is removed during iteration,
the corresponding iteration value will not be produced. If a map entry is
created during iteration, that entry may be produced during the iteration or
may be skipped. The choice may vary for each entry created and from one
iteration to the next.
If the map is nil, the number of iterations is 0.

For channels, the iteration values produced are the successive values sent on
the channel until the channel is closed. If the channel
is nil, the range expression blocks forever.

The iteration values are assigned to the respective
iteration variables as in an assignment statement.

The iteration variables may be declared by the "range" clause using a form of
short variable declaration
(:=).
In this case their types are set to the types of the respective iteration values
and their scope is the block of the "for"
statement; they are re-used in each iteration.
If the iteration variables are declared outside the "for" statement,
after execution their values will be those of the last iteration.

Go statements

The expression must be a function or method call; it cannot be parenthesized.
Calls of built-in functions are restricted as for
expression statements.

The function value and parameters are
evaluated as usual
in the calling goroutine, but
unlike with a regular call, program execution does not wait
for the invoked function to complete.
Instead, the function begins executing independently
in a new goroutine.
When the function terminates, its goroutine also terminates.
If the function has any return values, they are discarded when the
function completes.

A case with a RecvStmt may assign the result of a RecvExpr to one or
two variables, which may be declared using a
short variable declaration.
The RecvExpr must be a (possibly parenthesized) receive operation.
There can be at most one default case and it may appear anywhere
in the list of cases.

Execution of a "select" statement proceeds in several steps:

For all the cases in the statement, the channel operands of receive operations
and the channel and right-hand-side expressions of send statements are
evaluated exactly once, in source order, upon entering the "select" statement.
The result is a set of channels to receive from or send to,
and the corresponding values to send.
Any side effects in that evaluation will occur irrespective of which (if any)
communication operation is selected to proceed.
Expressions on the left-hand side of a RecvStmt with a short variable declaration
or assignment are not yet evaluated.

If one or more of the communications can proceed,
a single one that can proceed is chosen via a uniform pseudo-random selection.
Otherwise, if there is a default case, that case is chosen.
If there is no default case, the "select" statement blocks until
at least one of the communications can proceed.

Unless the selected case is the default case, the respective communication
operation is executed.

If the selected case is a RecvStmt with a short variable declaration or
an assignment, the left-hand side expressions are evaluated and the
received value (or values) are assigned.

The statement list of the selected case is executed.

Since communication on nil channels can never proceed,
a select with only nil channels and no default case blocks forever.

The expression list in the "return" statement may be a single
call to a multi-valued function. The effect is as if each value
returned from that function were assigned to a temporary
variable with the type of the respective value, followed by a
"return" statement listing these variables, at which point the
rules of the previous case apply.

func complexF2() (re float64, im float64) {
return complexF1()
}

The expression list may be empty if the function's result
type specifies names for its result parameters.
The result parameters act as ordinary local variables
and the function may assign values to them as necessary.
The "return" statement returns the values of these variables.

Regardless of how they are declared, all the result values are initialized to
the zero values for their type upon entry to the
function. A "return" statement that specifies results sets the result parameters before
any deferred functions are executed.

Implementation restriction: A compiler may disallow an empty expression list
in a "return" statement if a different entity (constant, type, or variable)
with the same name as a result parameter is in
scope at the place of the return.

Defer statements

A "defer" statement invokes a function whose execution is deferred
to the moment the surrounding function returns, either because the
surrounding function executed a return statement,
reached the end of its function body,
or because the corresponding goroutine is panicking.

The expression must be a function or method call; it cannot be parenthesized.
Calls of built-in functions are restricted as for
expression statements.

Each time a "defer" statement
executes, the function value and parameters to the call are
evaluated as usual
and saved anew but the actual function is not invoked.
Instead, deferred functions are invoked immediately before
the surrounding function returns, in the reverse order
they were deferred.
If a deferred function value evaluates
to nil, execution panics
when the function is invoked, not when the "defer" statement is executed.

For instance, if the deferred function is
a function literal and the surrounding
function has named result parameters that
are in scope within the literal, the deferred function may access and modify
the result parameters before they are returned.
If the deferred function has any return values, they are discarded when
the function completes.
(See also the section on handling panics.)

Built-in functions

Built-in functions are
predeclared.
They are called like any other function but some of them
accept a type instead of an expression as the first argument.

The built-in functions do not have standard Go types,
so they can only appear in call expressions;
they cannot be used as function values.

Close

For a channel c, the built-in function close(c)
records that no more values will be sent on the channel.
It is an error if c is a receive-only channel.
Sending to or closing a closed channel causes a run-time panic.
Closing the nil channel also causes a run-time panic.
After calling close, and after any previously
sent values have been received, receive operations will return
the zero value for the channel's type without blocking.
The multi-valued receive operation
returns a received value along with an indication of whether the channel is closed.

Length and capacity

The built-in functions len and cap take arguments
of various types and return a result of type int.
The implementation guarantees that the result always fits into an int.

The capacity of a slice is the number of elements for which there is
space allocated in the underlying array.
At any time the following relationship holds:

0 <= len(s) <= cap(s)

The length of a nil slice, map or channel is 0.
The capacity of a nil slice or channel is 0.

The expression len(s) is constant if
s is a string constant. The expressions len(s) and
cap(s) are constants if the type of s is an array
or pointer to an array and the expression s does not contain
channel receives or (non-constant)
function calls; in this case s is not evaluated.
Otherwise, invocations of len and cap are not
constant and s is evaluated.

Allocation

The built-in function new takes a type T,
allocates storage for a variable of that type
at run time, and returns a value of type *Tpointing to it.
The variable is initialized as described in the section on
initial values.

new(T)

For instance

type S struct { a int; b float64 }
new(S)

allocates storage for a variable of type S,
initializes it (a=0, b=0.0),
and returns a value of type *S containing the address
of the location.

Making slices, maps and channels

The built-in function make takes a type T,
which must be a slice, map or channel type,
optionally followed by a type-specific list of expressions.
It returns a value of type T (not *T).
The memory is initialized as described in the section on
initial values.

Each of the size arguments n and m must be of integer type
or an untyped constant.
A constant size argument must be non-negative and representable
by a value of type int; if it is an untyped constant it is given type int.
If both n and m are provided and are constant, then
n must be no larger than m.
If n is negative or larger than m at run time,
a run-time panic occurs.

Calling make with a map type and size hint n will
create a map with initial space to hold n map elements.
The precise behavior is implementation-dependent.

Appending to and copying slices

The built-in functions append and copy assist in
common slice operations.
For both functions, the result is independent of whether the memory referenced
by the arguments overlaps.

The variadic function append
appends zero or more values x
to s of type S, which must be a slice type, and
returns the resulting slice, also of type S.
The values x are passed to a parameter of type ...T
where T is the element type of
S and the respective
parameter passing rules apply.
As a special case, append also accepts a first argument
assignable to type []byte with a second argument of
string type followed by .... This form appends the
bytes of the string.

append(s S, x ...T) S // T is the element type of S

If the capacity of s is not large enough to fit the additional
values, append allocates a new, sufficiently large underlying
array that fits both the existing slice elements and the additional values.
Otherwise, append re-uses the underlying array.

The function copy copies slice elements from
a source src to a destination dst and returns the
number of elements copied.
Both arguments must have identical element type T and must be
assignable to a slice of type []T.
The number of elements copied is the minimum of
len(src) and len(dst).
As a special case, copy also accepts a destination argument assignable
to type []byte with a source argument of a string type.
This form copies the bytes from the string into the byte slice.

Deletion of map elements

The built-in function delete removes the element with key
k from a mapm. The
type of k must be assignable
to the key type of m.

delete(m, k) // remove element m[k] from map m

If the map m is nil or the element m[k]
does not exist, delete is a no-op.

Manipulating complex numbers

Three functions assemble and disassemble complex numbers.
The built-in function complex constructs a complex
value from a floating-point real and imaginary part, while
real and imag
extract the real and imaginary parts of a complex value.

The type of the arguments and return value correspond.
For complex, the two arguments must be of the same
floating-point type and the return type is the complex type
with the corresponding floating-point constituents:
complex64 for float32 arguments, and
complex128 for float64 arguments.
If one of the arguments evaluates to an untyped constant, it is first
converted to the type of the other argument.
If both arguments evaluate to untyped constants, they must be non-complex
numbers or their imaginary parts must be zero, and the return value of
the function is an untyped complex constant.

For real and imag, the argument must be
of complex type, and the return type is the corresponding floating-point
type: float32 for a complex64 argument, and
float64 for a complex128 argument.
If the argument evaluates to an untyped constant, it must be a number,
and the return value of the function is an untyped floating-point constant.

The real and imag functions together form the inverse of
complex, so for a value z of a complex type Z,
z == Z(complex(real(z), imag(z))).

If the operands of these functions are all constants, the return
value is a constant.

Handling panics

While executing a function F,
an explicit call to panic or a run-time panic
terminates the execution of F.
Any functions deferred by F
are then executed as usual.
Next, any deferred functions run by F's caller are run,
and so on up to any deferred by the top-level function in the executing goroutine.
At that point, the program is terminated and the error
condition is reported, including the value of the argument to panic.
This termination sequence is called panicking.

panic(42)
panic("unreachable")
panic(Error("cannot parse"))

The recover function allows a program to manage behavior
of a panicking goroutine.
Suppose a function G defers a function D that calls
recover and a panic occurs in a function on the same goroutine in which G
is executing.
When the running of deferred functions reaches D,
the return value of D's call to recover will be the value passed to the call of panic.
If D returns normally, without starting a new
panic, the panicking sequence stops. In that case,
the state of functions called between G and the call to panic
is discarded, and normal execution resumes.
Any functions deferred by G before D are then run and G's
execution terminates by returning to its caller.

The return value of recover is nil if any of the following conditions holds:

panic's argument was nil;

the goroutine is not panicking;

recover was not called directly by a deferred function.

The protect function in the example below invokes
the function argument g and protects callers from
run-time panics raised by g.

Bootstrapping

Current implementations provide several built-in functions useful during
bootstrapping. These functions are documented for completeness but are not
guaranteed to stay in the language. They do not return a result.

Function Behavior
print prints all arguments; formatting of arguments is implementation-specific
println like print but prints spaces between arguments and a newline at the end

Implementation restriction: print and println need not
accept arbitrary argument types, but printing of boolean, numeric, and string
types must be supported.

Packages

Go programs are constructed by linking together packages.
A package in turn is constructed from one or more source files
that together declare constants, types, variables and functions
belonging to the package and which are accessible in all files
of the same package. Those elements may be
exported and used in another package.

Source file organization

Each source file consists of a package clause defining the package
to which it belongs, followed by a possibly empty set of import
declarations that declare packages whose contents it wishes to use,
followed by a possibly empty set of declarations of functions,
types, variables, and constants.

A set of files sharing the same PackageName form the implementation of a package.
An implementation may require that all source files for a package inhabit the same directory.

Import declarations

An import declaration states that the source file containing the declaration
depends on functionality of the imported package
(§Program initialization and execution)
and enables access to exported identifiers
of that package.
The import names an identifier (PackageName) to be used for access and an ImportPath
that specifies the package to be imported.

The PackageName is used in qualified identifiers
to access exported identifiers of the package within the importing source file.
It is declared in the file block.
If the PackageName is omitted, it defaults to the identifier specified in the
package clause of the imported package.
If an explicit period (.) appears instead of a name, all the
package's exported identifiers declared in that package's
package block will be declared in the importing source
file's file block and must be accessed without a qualifier.

The interpretation of the ImportPath is implementation-dependent but
it is typically a substring of the full file name of the compiled
package and may be relative to a repository of installed packages.

Implementation restriction: A compiler may restrict ImportPaths to
non-empty strings using only characters belonging to
Unicode's
L, M, N, P, and S general categories (the Graphic characters without
spaces) and may also exclude the characters
!"#$%&'()*,:;<=>?[\]^`{|}
and the Unicode replacement character U+FFFD.

Assume we have compiled a package containing the package clause
package math, which exports function Sin, and
installed the compiled package in the file identified by
"lib/math".
This table illustrates how Sin is accessed in files
that import the package after the
various types of import declaration.

An import declaration declares a dependency relation between
the importing and imported package.
It is illegal for a package to import itself, directly or indirectly,
or to directly import a package without
referring to any of its exported identifiers. To import a package solely for
its side-effects (initialization), use the blank
identifier as explicit package name:

import _ "lib/math"

An example package

Here is a complete Go package that implements a concurrent prime sieve.

Program initialization and execution

The zero value

When storage is allocated for a variable,
either through a declaration or a call of new, or when
a new value is created, either through a composite literal or a call
of make,
and no explicit initialization is provided, the variable or value is
given a default value. Each element of such a variable or value is
set to the zero value for its type: false for booleans,
0 for numeric types, ""
for strings, and nil for pointers, functions, interfaces, slices, channels, and maps.
This initialization is done recursively, so for instance each element of an
array of structs will have its fields zeroed if no value is specified.

These two simple declarations are equivalent:

var i int
var i int = 0

After

type T struct { i int; f float64; next *T }
t := new(T)

the following holds:

t.i == 0
t.f == 0.0
t.next == nil

The same would also be true after

var t T

Package initialization

Within a package, package-level variables are initialized in
declaration order but after any of the variables
they depend on.

More precisely, a package-level variable is considered ready for
initialization if it is not yet initialized and either has
no initialization expression or
its initialization expression has no dependencies on uninitialized variables.
Initialization proceeds by repeatedly initializing the next package-level
variable that is earliest in declaration order and ready for initialization,
until there are no variables ready for initialization.

If any variables are still uninitialized when this
process ends, those variables are part of one or more initialization cycles,
and the program is not valid.

The declaration order of variables declared in multiple files is determined
by the order in which the files are presented to the compiler: Variables
declared in the first file are declared before any of the variables declared
in the second file, and so on.

Dependency analysis does not rely on the actual values of the
variables, only on lexical references to them in the source,
analyzed transitively. For instance, if a variable x's
initialization expression refers to a function whose body refers to
variable y then x depends on y.
Specifically:

A reference to a variable or function is an identifier denoting that
variable or function.

A reference to a method m is a
method value or
method expression of the form
t.m, where the (static) type of t is
not an interface type, and the method m is in the
method set of t.
It is immaterial whether the resulting function value
t.m is invoked.

A variable, function, or method x depends on a variable
y if x's initialization expression or body
(for functions and methods) contains a reference to y
or to a function or method that depends on y.

Dependency analysis is performed per package; only references referring
to variables, functions, and methods declared in the current package
are considered.

For example, given the declarations

var (
a = c + b
b = f()
c = f()
d = 3
)
func f() int {
d++
return d
}

the initialization order is d, b, c, a.

Variables may also be initialized using functions named init
declared in the package block, with no arguments and no result parameters.

func init() { … }

Multiple such functions may be defined per package, even within a single
source file. In the package block, the init identifier can
be used only to declare init functions, yet the identifier
itself is not declared. Thus
init functions cannot be referred to from anywhere
in a program.

A package with no imports is initialized by assigning initial values
to all its package-level variables followed by calling all init
functions in the order they appear in the source, possibly in multiple files,
as presented to the compiler.
If a package has imports, the imported packages are initialized
before initializing the package itself. If multiple packages import
a package, the imported package will be initialized only once.
The importing of packages, by construction, guarantees that there
can be no cyclic initialization dependencies.

Package initialization—variable initialization and the invocation of
init functions—happens in a single goroutine,
sequentially, one package at a time.
An init function may launch other goroutines, which can run
concurrently with the initialization code. However, initialization
always sequences
the init functions: it will not invoke the next one
until the previous one has returned.

To ensure reproducible initialization behavior, build systems are encouraged
to present multiple files belonging to the same package in lexical file name
order to a compiler.

Program execution

A complete program is created by linking a single, unimported package
called the main package with all the packages it imports, transitively.
The main package must
have package name main and
declare a function main that takes no
arguments and returns no value.

func main() { … }

Program execution begins by initializing the main package and then
invoking the function main.
When that function invocation returns, the program exits.
It does not wait for other (non-main) goroutines to complete.

Errors

The predeclared type error is defined as

type error interface {
Error() string
}

It is the conventional interface for representing an error condition,
with the nil value representing no error.
For instance, a function to read data from a file might be defined:

func Read(f *File, b []byte) (n int, err error)

Run-time panics

Execution errors such as attempting to index an array out
of bounds trigger a run-time panic equivalent to a call of
the built-in function panic
with a value of the implementation-defined interface type runtime.Error.
That type satisfies the predeclared interface type
error.
The exact error values that
represent distinct run-time error conditions are unspecified.

System considerations

Package unsafe

The built-in package unsafe, known to the compiler
and accessible through the import path"unsafe",
provides facilities for low-level programming including operations
that violate the type system. A package using unsafe
must be vetted manually for type safety and may not be portable.
The package provides the following interface:

A Pointer is a pointer type but a Pointer
value may not be dereferenced.
Any pointer or value of underlying typeuintptr can be converted to
a type of underlying type Pointer and vice versa.
The effect of converting between Pointer and uintptr is implementation-defined.

The functions Alignof and Sizeof take an expression x
of any type and return the alignment or size, respectively, of a hypothetical variable v
as if v was declared via var v = x.

The function Offsetof takes a (possibly parenthesized) selectors.f, denoting a field f of the struct denoted by s
or *s, and returns the field offset in bytes relative to the struct's address.
If f is an embedded field, it must be reachable
without pointer indirections through fields of the struct.
For a struct s with field f:

Computer architectures may require memory addresses to be aligned;
that is, for addresses of a variable to be a multiple of a factor,
the variable's type's alignment. The function Alignof
takes an expression denoting a variable of any type and returns the
alignment of the (type of the) variable in bytes. For a variable
x:

uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0

Calls to Alignof, Offsetof, and
Sizeof are compile-time constant expressions of type uintptr.